Fix missing functionality with --allow-native-extensions

pkg/compiler/lib/src/native/native.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library native;
 
 import '../compiler.dart' show Compiler;
 import '../elements/elements.dart';
 
 export 'behavior.dart';
@@ skipping 20 matching lines @@
     String scriptName = library.entryCompilationUnit.script.name;
     return scriptName.contains('sdk/tests/compiler/dart2js_native') ||
         scriptName.contains('sdk/tests/compiler/dart2js_extra');
   }
   bool allowedDartLibary() {
     Uri uri = library.canonicalUri;
     if (uri.scheme != 'dart') return false;
     return _allowedDartSchemePaths.contains(uri.path);
   }
 
-  return allowedTestLibrary() || allowedDartLibary();
+  return allowedTestLibrary() ||
+      allowedDartLibary() ||
+      compiler.options.allowNativeExtensions;
 }